This invention relates to testing and characterization of low noise microwave and RF transistor chips on wafer (device under test, DUT); the method disclosed comprises a test setup and an algorithm for data acquisition allowing extraction of the “four noise parameters” of the DUT. The test setup uses automatic microwave tuners to synthesize reflection factors (or impedances) at the input of the DUT and high sensitivity (noise) receivers for collecting the necessary data using appropriate computer software and control.
All RF two-ports using semiconductor devices (DUT) contain internal noise sources which affect the purity of the signal entering at the input port and exiting (amplified) at the output port. A common way of characterizing the “purity” of the DUT at each frequency and bias condition is the noise figure: F. One definition of the noise figure is the degradation of the signal to noise ratio (S/N) between the input and output port of the DUT expressed as the input to output signal to noise ratio: F=(S.in/N.in)/(S.out/N.out) {eq. 1}, whereby S.in and S.out are the signal power levels at the input and output of the DUT and N.in and N.out the corresponding noise power levels. Since the DUT adds to the transmitted signal its internal noise, the S/N ratio at the input is higher than at the output, therefore F>1.
It has been established (see ref. 1) that four real numbers fully describe the noise behavior of any linear noisy two-port; these are the four noise parameters. By, generally accepted convention, the four noise parameters (4NP) are: Minimum Noise Figure (Fmin), Equivalent Noise Resistance (Rn) and Optimum Noise Admittance (Yopt=Gopt+j*Bopt) (see ref. 1). The noise behavior of a two-port only depends on the admittance of the source and not of the load. The general relationship is: F(Ys)=Fmin+Rn/Re(Ys)*|Ys−YoptI2 {eq. 2}.
F(Ys) in eq. 2 being the noise figure F.total of the chain including the DUT any following hardware, like bias tees and RF switches and the receiver (FIG. 5), the natural law of cascaded noisy two-ports established by FRIIS (see ref. 2) is used to extract the noise figure of the DUT itself: FRIIS' formula is: F.dut=F.total−(F.rec−1)/Gav.dut(Sij) {eq. 3}; hereby F.dut is the noise figure of the DUT, F.rec is the noise figure of the receiver and Gav.dut is the available Gain of the DUT for a given frequency and associated bias conditions. Whereas F.total can be measured directly (see ref. 6) F.rec(Ys) and Gav.dut(Ys, Sij) depend both, (a) on the small signal properties of the DUT (see ref. 3), which are customarily described using the s-parameters, and (b) on the source admittance Ys as per eq. 2; Sij are the DUT s-parameters (see ref. 3). In eq. 3 the available gain Gav.dut of the DUT can only be calculated using Ys and the DUT s-parameters; these s-parameters must be accurate and measured, if possible, immediately before the noise data acquisition, to avoid device drifting, and allow calculations using eq. 3. This is the reason for using RF switches in the measurement path (see ref. 4, items 54 and 64 in FIG. 1 and FIG. 5).
A commonly used prior art test setups are shown in FIG. 1 (see ref. 5); the test system comprises: a calibrated noise source (52), an impedance tuner (60), a test fixture (10) to hold the DUT, a sensitive noise receiver (72) and two SPDT RF switches (54 and 64). The tuner (60) and the noise receiver (72) are controlled by a system computer (not shown), which sets the source admittance Ys, created by the tuner, and retrieves digitally the associated noise measurement data from the noise receiver (72). S-parameters are measured by toggling the RF switches towards the VNA (70). After termination of the measurement session the computer program processes the measured data using above equations and extracts the four noise parameters of the DUT for a given frequency and DUT bias conditions.
There are some endemic problems with RF switches (FIGS. 2 to 4): If they are electronic (based on PIN diodes, see ref. 7) they have limited band width and, especially, high insertion loss (see “insertion loss” in ref. 7); if they are mechanical they have uncertain and often limited repeatability (FIGS. 4A and 4B). An even more important limitation is availability. Today there exist no coaxial RF switches operating above 67 GHz. There exist waveguide switches reaching above 110 GHz, but they are limited in the waveguide bands (50-75 GHz, 60-90 GHz, 75-110 GHz). To configure a 50-110 GHz noise measurement system, for instance, one needs alternative solutions. This is what is proposed in this invention disclosing a test setup that performs the same tasks as the traditional setup without using RF switches.